【单选题】

1The human ear contains the organ for hearing and the organ for balance.Both organs involve fluid-filled channels containing hair cells that produce electrochemical impulses when the hairs are stimulated by moving fluiD、

2The ear can be divided into three regions: outer, middle, and inner. The outer ear collects sound waves and directs them to the eardrum separating the outer ear from the middle ear. The middle ear conducts sound vibrations through three small bones to the inner ear. The inner ear is a network of channels containing fluid that moves in response to sound or movement.
3To perform the function of hearing, the ear converts the energy of pressure waves moving through the air into nerve impulses that the brain perceives as sounD、Vibrating objects, such as the vocal cords of a speaking person, create waves in the surrounding air. These waves cause the eardrum to vibrate with the same frequency. The three bones of the middle ear amplify and transmit the vibrations to the oval window, a membrane on the surface of the cochlea, the organ of hearing. Vibrations of the oval window produce pressure waves in the fluid inside the cochleA、Hair cells in the cochlea convert the energy of the vibrating fluid into impulses that travel along the auditory nerve to the brain.
4The organ for balance is also located in the inner ear. Sensations related to body position are generated much like sensations of sounD、Hair cells in the inner ear respond to changes in head position with respect to gravity and movement. Gravity is always pulling down on the hairs, sending a constant series of impulses to the brain. When the position of the head changes--as when the head bends forward--the force on the hair cells changes its output of nerve impulses. The brain then interprets these changes to determine the head’s new position.
What can be inferred from paragraph 4 about gravityA．Gravity has an essential role in the sense of balance.
B．The ear converts gravity into sound waves in the air.
C．Gravity is a force that originates in the human ear.
D．The organ for hearing is not subject to gravity.